In producing aspherical lenses, a blank is manufactured with a shape generally corresponding to the final shape of the lens. This blank is mounted in a lathe or the like, and different tools are applied to the surface of the blank for machining its surface to its final form. The tools are subjected to a certain amount of wear, and the blank is subjected to the effect of forces, which result in an elastic deformation of the blank. This leads to machining errors. Each tool must have a tip configuration suited to the intended surface curvature in the respective machined partial area of the lens/mould. The tools must be sharpened. Friction between tool and blank causes vibrations which reduce the surface finish. All this leads to high machining costs and insufficient surface finish.
There is also substantial interest in producing aspherically corrected fresnel-type lenses, and especially with stepped surfaces which form an angle to the axis of the lens surface. Such lenses are extremely difficult to produce with current production methods.
There are corresponding problems in the production of moulds or dies with which aspherically corrected lenses of the kind mentioned above can be produced in a compression or ejection moulding operation. Such moulds usually consist of metal. The demands on the mould surface are naturally high also, and to reduce to some extent the disadvantages which are associated with the direct production of glass lenses of the kind in question, it has been proposed to utilize electrical-discharge machining of the metal mould surface. However, it has been found that electrical-discharge machining results in very small surface irregularities, which are less desirable. Moulds machined in this way are given a surface which looks like that of orange peel, when seen through a microscope.
It has therefore been considered necessary to keep to the turning technique in the direct production of glass lenses as well as moulds for compression or ejection moulding of lenses, particularly plastics lenses.
DE-OS No. 2,804,479 reveals a method of machining a workpiece wherein the workpiece, which has a polygonal periphery, is rotated and a laser beam is directed tangentially to the periphery in order to form a circular groove therein. The object is to avoid shock loads by the workpiece corners on a conventional tool applied against the periphery.
The present invention is not concerned with the problem of avoiding shock loads by the work piece on a tool but pertains to elimination of surface irregularities on lenses or lens moulds which would occur in conventional turning technique due to tool wear, vibration and misalingments in connection with tool exchange.
One object of the invention is therefore to provide a method offering up-to-standard surface accuracy for lenses or lens moulds.